Investigation of the Sulfur Redox Reaction Mechanism by the Quantitative and Qualitative Measurement of Dissolved Polysulfide Ions

Deyang QuJohnson Control Endowed Chair Professor

College of Engineering and Applied Science, University of Wisconsin Milwaukee

Li‐SM3 Imperial College, London, UK

P.G. Bruce, Nature Material 11(2012)19

What Chemistry beyond Li‐ion?

The barriers for the Li‐S Chemistry

• The high resistance of sulfur.• Soluble polysulfides during charge and discharge• S cathode morphology changes.• Irreversible Li2S deposition on Li anode.

• Shuttle effect• Self‐discharge.

• The Li anode.• The electrolyte

Soluble Polysulfides Play Important Roles

P. Bruce etc. Nat. Mater. 11(2012)19

J.Xiao etc. JEC 162(2015)A474

C. Barchase Anal. Chem. 84 2012 3973

Questions:

1. Multistep multielectronelectrochemical process, why are there only two steps in the discharge curve?

2. How to quantitatively and qualitatively measure polysulfide species?

Quantitative and Qualitative Determination of Polysulfide Becomes Critical

R. Dominko etc. ChemSusChem 7(2014)2167 M Hagen etc. J. Electrochem. Soc. 160(2013)A1205

H.D. Abruna etc. RSC Adv. 4(2014)18347

UV‐Vis. Raman X‐Ray Absorption Near Edge

A accurate and precise determination of polysulfides cannot be done by the above analytical techniques, they can provide estimation at the best.

Why are the polysulfide ions so difficult to measured? – Lack of reference.

• It is impossible to make pure polysulfide ion with precise chain length in solution due to the chemical equilibrium and disproportionation.

• Mass Spectroscopy relies on mass/charge ratio, so pure reference is not needed. Therefore, by means of HPLC/MS, each individual polysulfide ions could be separated in the HPLC column and identified by MS,

HPLC‐UV is the technique NOT based on standards for quantitative and qualitative analysis

2 4 6 8 10 12 140.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

Elemental Sulfur

Inte

nsity

Retention Time (min)

Luna C18

Symmetry C18

Zorbax C8

Xterra MS C8

2 4 6 8 10 12

0.0

2.0x104

4.0x104

6.0x104

8.0x104

0.0 0.5 1.0 1.5 2.00.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

y=12615x-84.197R2=0.9999Sy(standard error) =0.00782

Peak

Are

a of

Sul

fur

Concentration (mM)

Elemental Sulfur

Inte

nsity

Retention Time (min)

1.889 mM

0.378 mM

0.0756 mM

0.0151 mM

Salt Solubility (mM) LiTFSi 1.933 LiClO4 2.416 LiBF4 1.768 LiCF3SO3 2.161 TEABF4 2.170

 

Solvent Solubility mM, pure solvent

Solubility (mM), 0.1M electrolyte (LiTFSI)

Solubility (mM), in 1.0M electrolyte

AN 0.610 0.596 0.390 PY 48.046 28.005 15.909 DMF 5.944 5.895 2.603 PC 1.318 1.255 0.633 GBL 3.888 3.366 1.606 DGME 10.259 9.511 3.875 DME 9.957 8.963 3.994 DMSO 3.936 3.845 1.933 BMPTFSi 0.349 0.245 0.216  

• Elemental Sulfur (S8) can be separated and detected.

• The solubility of S8 is different in different solvents.

• The solubility of S8 changes with salt in the same solvent.

Solubility of S8 in pure solvent

Solubility of S8 in DMSO with different Salts (1M)

S8 analysis

D.Y. Qu etc. J. Electrochem. Soc., 162(2015)A203

Polysulfides itself cannot be separated in LC column

• Solutions:– A:pure DME– B:Na2 S saturated DME– C: S8 saturated DME– D:DME saturated with both S8 and Na2S

• Only S8 can be detected by UV and only DME can be detected by ESI‐MS

• Polysulfides cannot be separated due lack of retention mechanism.

2 4 6 8 10

0.0

5.0x107

1.0x108

1.5x108

2.0x108

A

B

C

D

Retention Time (min)

Inte

nsity

2 4 6 8 10 12

0.0

2.0x104

4.0x104

6.0x104

8.0x104

Retention Time (min)

Inte

nsity

LC/UV

LC/MS

1.67 min

3.085 min

7.012 min

D.Y. Qu etc. J. Electrochem. Soc., 162(2015)A203

Polysulfides can be separated and analysis by MS with complexiationessay 

Na2S and S8in acetonitrile, molar ratio of Na2S:S is 1:3

ALL 8 polysulfides are separated and qualitatively analyzed for the first

D.Y Qu etc, Adv. Energy Mater. 5(2015)1401888

Discharge of a sulfur cathode

10

0 200 400 600 800 1000 1200

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.73

5 V

1.83

3 V

1.99

4 V

2.04

7 V

2.06

8 V

2.09

1 V

2.15

0 V

2.20

7 V

2.26

6 V

2.40

1 V

2.43

5 V

RS3R RS4R RS5R RS6R RS7R RS8R S8

S2-7

S2-6 S2-

5 S2-4 S2-

3 S2-2

Rel

ativ

e P

eak

Are

a

Specific Capacity (mAh*g-1)

S2-8

>2.3 V, first plateau, major species are bold

⇄ ⇄

Between 2.3 and 2.1 V, major species are bold

↓ ↓

<2.1 plateau, major species are bold

↓ ↓

⇄ ⇄

Two major chemical equilibriums  ‐ two discharge plateaus.

Recharge an process

11

0 200 400 600 800 1000 1200 1400

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

2.90

2 V

2.50

4 V

2.46

1 V

2.42

0 V

2.40

1 V

2.37

5 V

2.35

1 V

2.32

5 V

2.30

0 V

2.31

3 V

1.73

5 V

RS3R RS4R RS5R RS6R RS7R RS8R S8

Rel

ativ

e Pe

ak A

rea

Specific Capacity (mAh*g-1)

Cathode potential <2.4 V

↓ ↓

⇄ ⇄ ⇄

Cathode potential > 2.4 V

⇄ ⇄ ⇄

1. An single chemical equilibrium in the recharge.

2. Sulfur redox reaction is “reversible”.

Sulfur reduction to polysulfide

Scheme 1:

Scheme 2:

Scheme 3:

Polysulides with chain length of 8 were proposed in the mechanisms reported in the literature.

In‐situ capture of polysulfide formed during reduction

Catholyte A :0.0194 g sulfur in 20 mL 1M LiTFSi/DME solution saturated Li2S. Glass carbon electrode; 30 mV/s

• To investigate the mechanism of sulfur reduction, the polysulfidesformed during electrochemical reaction need to be identified before subsequent CHEMICAL reaction in the electrolyte.

• When the polysulfide ions are derivatized, the resulting complexes are inert – not participate further chemical reaction nor be electrochemically reduced or oxidized.

Qu, D etc J Power Sources 301(2016)312

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Curve 1: 20 mM methy triflate.

Curve 2: catholyte A

Curve 4: S8 saturated with 20 mM methyl triflate

Cur

rent

Den

sity

/ m

Acm

-2

Potential / V vs. Li+/Li)

Curve 3: S8 saturated

Comparison of in‐situ and ex‐situ detection methods.

• The glass carbon electrode was polarized at 2.3 V vs. Li – the first electron transfer (from elemental sulfur to polysulfide.

• Black: ex‐situ (electrolyte was taken out and then derivatizedoutside)

• Red: in‐situ (derivatizer was added into the catholytesolution)

• S82‐ was not the polysulfide form in the electrochemical reduction. 

Sulfur saturated in 20 mL 1M LiTFSi/DME, Glass carbon electrode; polarized at 2.3 V vs Li

The electrochemically formed polysulfides react with elemental sulfur chemically

Derivatizationmethod

Discharge capacity at 2.3V (mAh)

Percentage of elemental sulfur left from theoretical calculation based on 2-electron transfer

Percentage of elemental sulfur left from HPLC/UV measurement

In-Situ 1.42 34.0% 37.9%

Ex-Situ 1.43 33.3% 24.9%

Electrochemical reduction of elemental sulfur was not enough to count for all the sulfur consumption in Li‐S cell.

S82‐ is NOT Electrochemically formed

As soon as polysulfide ions are formed electrochemically, they will be captured by derivatization, the subsequent chemical reactions are avoided. The method provides a “snap shoot”.

Conclusion

• Essays are developed to quantitatively and qualitatively determine the polysulfide ions during the operation of Li‐S batteries.

• All the soluble elemental sulfur and polysulfide ions can be separated and qualitatively determined by HPLC‐UV/MS with proper treatment.

• The technique offers a powerful tool for the investigation of S redox mechanism.

• The initial reduction product from elemental sulfur is not polysulfide with the chain length of 8.

Acknowledgment 

We are very grateful for the continuous financial support from the Department of Energy, EERE, OVT, BMR program.

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